Generalize variables and function names used for planes composition
(from cursor to plane), since we will reuse the operations for both
cursor and overlay types.

No functional change.

v4:
- use better names for functions of plane composition (Daniel)
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/ef3249c9ac3a08b7079894f9f1ccf70960194ec7.1619250933.git.melissa.srw@gmail.com